@article{MS2_1970__26_166130,
     author = {Tomas Claesson},
     title = {Partial {Hypoellipticity} and {Gevrey} {Classes.}},
     journal = {Mathematica Scandinavica},
     pages = {119--131},
     year = {1970},
     volume = {26},
     zbl = {0201.12502},
     url = {http://geodesic.mathdoc.fr/item/MS2_1970__26_166130/}
}